Rocket motor insulation containing hydrophobic particles

ABSTRACT

A rocket motor insulation including an elastomer base polymer is improved in its processability by the addition of silica particles treated with a hydrophobic coating. The insulation also preferably includes a metallic coagent cross-linker, which when used in combination with the hydrophobic silica particles increases the tear strength and the elasticity of the insulation, while at the same time not adversely affecting the bonding characteristics of the insulation.

RELATED APPLICATIONS

The benefit of priority is claimed of provisional application 60/142,960filed in the U.S. Patent & Trademark Office on Jul. 12, 1999, thecomplete disclosure of which is incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention is directed to elastomer-based insulation for rocketmotors, such as the type interposed between a solid propellant grain anda rocket motor casing to protect the casing from high temperaturesexperienced during burning of the solid propellant grain. In particular,this invention is directed to a solid rocket motor insulationcomposition that is relatively insensitive to process variables such asmoisture contamination and relative humidity, yet upon curing exhibitsexcellent physical properties and thermal and ablative performances.

2. Description of the Related Art

Solid rocket motors typically include an outer casing or case housing asolid propellant grain. The rocket motor casing is conventionallymanufactured from a rigid, yet durable, material such as steel orfilament-wound composite. The propellant is housed within the casing andis formulated from a composition designed to undergo combustion whileproducing the requisite thrust for attaining rocket motor propulsion.

During operation, a heat insulating layer or layers (insulation)protects the rocket motor casing from heat and erosion caused byparticle streams generated by combustion of the propellant. Typically,the insulation is bonded to the inner surface of the casing and isgenerally fabricated from a composition that, upon curing, is capable ofwithstanding the high temperature gases and erosive particles producedwhile the propellant grain burns. A liner layer (liner) functions tobond the propellant grain to the insulating layer and to anynon-insulated portions of the casing. Liners also have an ablativefunction, inhibiting burning of the insulation at liner-to-insulationinterfaces. Liner compositions are generally known to those skilled inthe art. An exemplary liner composition and process for applying thesame is disclosed in U.S. Pat. No. 5,767,221, the disclosure of which isincorporated herein by reference.

The combustion of solid rocket propellant generates extreme conditionswithin the rocket motor casing. For example, temperatures inside therocket motor casing typically reach 2,760° C. (5,000° F.). These factorscombine to create a high degree of turbulence within the rocket motorcasing. In addition, the gases produced during propellant combustiontypically contain high-energy particles that, under a turbulentenvironment such as encountered in a rocket motor, can erode the rocketmotor insulation. If the propellant penetrates through the insulationand liner, the casing may melt, causing the rocket motor to fail. Thus,it is crucial that insulation withstands the extreme conditionsexperienced during propellant combustion and protects the casing fromthe burning propellant. Unless the insulation is capable of withstandingsuch conditions, failure may occur.

Further, once formulated but prior to full curing, the insulationcomposition must also possess acceptable shelf life characteristics suchthat the insulation composition remains sufficiently pliable until usedin application to the rocket motor casing. This requirement is essentialbecause the production of a given lot of insulation may have to wait instorage for a number of months prior to cure and installation.Similarly, after application to a rocket motor casing and subsequentcuring, a functionally acceptable solid propellant rocket motorinsulation must survive aging tests. Rocket motors may be fullyfabricated many months before actual firing; in the case of tacticalweapons especially, rocket motors may be fabricated as much as a yearbefore actual firing. Over that period of time, the insulationcomposition must continue to remain fully functional withoutunacceptable migration of its components to or from adjacent interfacialsurfaces and adequately retain its elastomeric characteristics toprevent brittleness. These requirements need to be satisfied underextremely wide temperature variations.

After application of the insulation to the interior of the rocket motorcasing, and subsequent to curing thereof, an acceptable cured insulationmust also exhibit satisfactory bonding characteristics to a variety ofadjacent surfaces. Such surfaces include the internal surface of therocket motor casing itself. The insulation must also exhibit adequatebonding characteristics with the propellant grain, or with a linersurface interposed between the insulation and propellant grain.

Further, cured insulation must meet the ablation limits for protectionof the rocket motor casing throughout the propellant burn without addingundue weight to the motor.

In the past, candidates for making rocket motor insulation have includedfilled and unfilled rubbers and plastics such as phenolic resins, epoxyresins, high temperature melamine-formaldehyde coatings, ceramics,polyester resins, and the like. The latter plastics, however, crackand/or blister as a result of the rapid temperature and pressurefluctuations experienced during combustion.

Elastomeric candidates have also been investigated and used. Theelastomers are used in a large number of rocket motors because theirthermal and ablative properties are particularly suited for rocket motorapplications. However, the mechanical properties of elastomers, such aselongation capabilities and tensile strength, are often inadequate forrocket motor operation and processing. For example, cured elastomericinsulation, whether thermosetting or thermoplastic, often becomesbrittle and cracks in operation unless reinforced with suitable fillers.The cracking of the cured elastomeric insulation creates paths throughthe insulation which expose the casing to the combustion reaction,thereby rendering the casing more susceptible to failure.

In order to improve the mechanical properties of elastomeric insulation,it has been proposed to reinforce the elastomeric insulation withprecipitated silica or silicate. The presence of precipitated silica orsilicate in elastomeric rocket motor insulation advantageously improvesthe mechanical properties of the elastomer matrix, and further has thesecondary benefit of improving the thermal and ablative performance ofthe insulation. The use of precipitated silica is reported, by way ofexample, in U.S. Pat. No. 5,498,649 to Guillot. However, because silicaand silicate particles are hydrophilic, insulation compositionscontaining precipitated silica and/or silicate are provided to absorbsignificant amounts of moisture when exposed to humid environments. Highmoisture content in a rocket motor insulation can adversely affectbonding characteristics of the insulation, especially at moisturesensitive interfaces, such as the insulation-to-casing bond interfaceand the insulation-to-liner bond interface. The later bond interface isparticularly sensitive to moisture because of the isocyanates typicallyused in liner formulations.

To address these problems, dry cycles have been implemented to controlthe moisture content during the manufacture of the insulation and whileinsulating the rocket motor case. However, the practice of theserequisite dry cycles complicates and prolongs processing. Thus, wherehydrophilic silica and/or silicate particles are used in insulationcompositions, very rigorous process controls commonly are imposed toaccount for process variables such as moisture contamination andrelative humidity.

SUMMARY OF THE INVENTION

It is, therefore, an object of this invention to provide a rocket motorinsulation composition that is relatively insensitive to processvariables such as moisture contamination and relative humidity, yet uponlaying-up into a rocket motor casing and subsequent curing exhibits andmaintains excellent low temperature and high temperature physicalproperties and thermal and ablative performances.

In accordance with the principles of this invention, the above and otherobjects are attained by a rocket motor composition comprising, prior tocuring into an elastomeric composition, at least one organic polymer, atleast one curative, optionally at least one curing co-agent, andhydrophilic particles coated with at least one hydrophobization agent.Preferably, the curative comprises one or more peroxides.

By using filler particles that have been treated with a suitablehydrophobization agent, the rocket motor composition exhibits reducedsensitivity to process variables such as moisture contamination andrelative humidity. Additionally, after peroxide curing in the presenceof the coagent, the elastomeric rocket motor insulator possessesexcellent insulating properties. To the surprise of the inventors,however, the elastomeric rocket motor insulator also exhibits improvedmechanical properties (e.g., elongation capability and tensile strength)over conventional peroxide-cured polymers containing hydrophilic silicaparticles. This finding of improved mechanical properties was surprisingand unexpected because hydrophobic silica particles evaluated by theinventors generally were believed to contribute less reinforcingcharacteristics to an elastomeric insulation than conventionalhydrophilic silica fillers. Although this invention is not intended tobe limited to any theory, the improvement in reinforcing characteristicscontributed by the hydrophobized silica particles is believed to be theresult of a synergistic effect realized by using the hydrophobizedparticles in combination with a peroxide curing agent and the coagentdescribed herein.

Other objects, aspects and advantages of the invention will be apparentto those skilled in the art upon reading the specification and appendedclaims which, when read in conjunction with the accompanying drawings,elucidate the principles of this invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings serve to elucidate the principles of thisinvention. In such drawings:

FIGS. 1A and 1B are schematic cross-sectional views of an embodiment ofa rocket motor assembly in which the insulation of this invention isprovided;

FIG. 2 is a graph showing the reduced moisture sensitivity of an exampleof the inventive insulation with reference to a comparative example;

FIG. 3 is a schematic cross-sectional view of a test char motor; and

FIG. 4 is a graph showing the reduced material loss in a thermal flashtest of an example of the inventive insulation compared to anothercomparative example.

DETAILED DESCRIPTION OF THE INVENTION

The present invention comprises rocket motor insulation, and acomposition curable into the rocket motor insulation. The insulationcomposition is commonly applied as a layer or layers into a rocket motorcasing 12, then is cured to form the insulation, which is generallydesignated in FIGS. 1A and 1B by reference numeral 10. Cast inside ofthe insulation 10 is a solid propellant 16, which is illustrated in FIG.1A as a center perforation propellant, although the invention is notthereby limited, since the inventive insulation may be used withend-burning propellants and other propellant configurations. Typically,a liner 14 is interposed between the insulation 10 and a solidpropellant 16, although the liner 14 may directly bond the propellant 16to the casing 12. The insulation 10 and liner 14 serve to protect thecasing 12 from the extreme conditions produced as the propellant 16 isburned. Methods for loading a rocket motor casing 12 with insulation 10,a liner 14, and propellant 16 are known to those skilled in the art, andcan be readily adapted within the skill of the art without undueexperimentation to incorporate the insulation composition of thisinvention. Nozzle 18 is operatively associated with the casing 12 toreceive combustion products generated by combustion of the propellant 16and to expel the combustion products, thus generating thrust to propelthe rocket.

As mentioned above, the inventive insulation contains, in a cured state,one or more organic elastomeric polymers. As referred to herein, theterm “organic elastomeric polymer” means a polymer having a backboneincluding carbon as a main component and free of metals or metalloids inthe backbone. Generally, an elastomeric polymer is stretchable andcompressible under moderate tension with a relatively high tensilestrength and memory so that, upon release of the tension or compression,the elastomer retracts towards its original dimensions. Organicelastomers suitable for the present invention includeethylene-propylene-diene monomer (EPDM) rubbers, natural rubber,butadiene-styrene copolymer rubbers, nitrile rubbers, polybutadienerubbers, polyisoprene rubbers, and the like. Various mixtures,combinations, copolymers, and blends of these exemplary rubbers are alsoincluded within the scope of the invention.

In the event that EPDM rubber is selected as the organic elastomer, itis advantageous to use an EPDM rubber having a high ethylene content,such as in the range of 50 to 70 percent by weight. EPDM polymers withrelatively high ethylene contents are known to enhance the greenstrength of uncured formulations. High green strength is important forfacilitating calendering operations during processing. Exemplary blendsof EPDM polymers include combinations of NORDEL® IP NDR-4520 and NORDEL®IP-NDR 4640 brand products, which each have an ethylene content in therange of 50 to 55% by weight. The NORDEL® IP-NDR 3722p brand product,which has an ethylene content of 70 percent by weight, is useful forincreasing the ethylene content to further improve green strength.

Preferably, the organic elastomeric polymers comprises from about 35 wt% to about 80 wt %, and still more preferably from about 45 wt % toabout 60 wt % of the total weight of the rocket motor insulation.

The peroxide generally functions as a crosslinking agent or promoter,for example, by abstracting the hydrocarbon atom from the elastomermolecule (e.g., the diene of the EPDM) and providing polymeric freeradicals for forming cross-links. The peroxide curative preferablycomprises from about 0.5 phr to about 8 phr, more preferably about 2 phrto about 5 phr, of the insulation composition. As referred to herein andgenerally accepted in the art, “phr” means parts by weight per onehundred parts by weight polymer. A representative, but not exhaustive orexclusive, list of suitable peroxide curatives includes dicumylperoxide, 2,5-dimethyl-2,5-bis-(t-butylperoxy)hexane,2,5-dimethyl-2,5-bis-(benzoylperoxy)hexane,2,5-dimethyl-2,5-di(t-butylperoxy)-3-hexane,n-butyl-4,4-bis-(t-butylperoxyl)valerate,4,4′-methylbis-(cyclohexylamine)carbomate,1,1-bis-(t-butylperoxy)-3,3,5-trimethylcyclohexane,α,α′-bis-(t-butylperoxy)diisopropylbenzene,2,5-dimethyl-2,5-bis-(t-butylperoxy)hexane, and di-t-butyl peroxide.Commercially available peroxide curatives are available, for example,under the trade name DI-CUP® 40KE, which comprises about 40% dicumylperoxide on a clay carrier. (The clay carrier is available from BurgessPigment Company.)

One or more curative coagents are preferably included to increase thedegree and rate of cure. Preferably, the curative coagent is an “ioniccurative coagent,” also referred to as a “metallic curative coagent,”which is meant to encompass a metal salt which is capable of forming anorganometallic crosslink bond having an ionic portion. Coagents includemetal salts of ethylenically unsaturated carboxylic acids, especiallythe metal salts of acrylic and methacrylic acids. A representative, butnot exhaustive or exclusive, list of ionic curative coagents includemetallic acrylates and methacrylates, such as zinc diacrylate and zincdimethacrylate, which are available from Sartomer Company as SARET® 633and SARET® 634, respectively. The use of zinc diacrylate and zincdimethacrylate as coagents in a much different environment is describedin U.S. Pat. No. 5,565,535, which is incorporated herein by reference.Generally, the ionic curative coagent is present in an effective amountto increase the degree of rate of cure, and representative amounts aregreater than 0 phr to 40 phr, more preferably 10 phr to 20 phr.Polyfunctional unsaturated organic compounds may also be selected as thecurative coagent. Suitable polyfunctional unsaturated organic compoundsinclude, by way of example, the following: methacrylates such astrimethylolpropane trimethacrylate, pentaerythritol trimethacrylate,pentaerythritol tetramethacrylate; acrylates such as pentaerythritoltriacrylate and trimethylolpropane triacrylate; imides such asN,N′-m-phenylene-dimaleimide; triallyl cyanurates; triallylisocyanurates; and diallyl phthalates.

Representative hydrophobization (or hydrophobic) agents include, by wayof example, the following: organohalosilanes, such asdimethyldichlorosilane, methyltrichlorosilane, dimethyldibromosilane,methyltribromosilane, diethyldichlorosilane, ethyltrichlorosilane,diproplydichlorosilane, diisopropyldichlorosilane,propyltrichlorosilane, dibutyldichlorosilane, and butyltrichlorosilane;disilazanes, such as hexamethyl-disilazane; organosilanes, such astrimethoxy-octyl-silane, hexadecyl silane, methyacryl-silane; siloxanessuch as octamethyl-cyclo-tetra-siloxane, and polydimethylsiloxane;compounds with one or more alkylsiloxyl moieties, such astrimethylsiloxyl moieties; or combinations thereof.

Methods of making various hydrophobization agents are disclosed in U.S.Pat. No. 4,072,796, the complete disclosure of which is incorporatedherein by reference. Hydrophobized silica is also commerciallyavailable. For example, silica particles treated withdimethyldichlorosilane are available from Degussa as AEROSIL® R972 andAEROSIL® R974, and are also available from Cabot Corporation asCAB-O-SIL® TS-610. Silica particles treated with hexamethyl-disilazaneare available from Degussa as AEROSIL® R812, AEROSIL® R812S, AEROSIL®R71 1, and AEROSIL® R8200, and also are available from Cabot Corporationas CAB-O-SIL® TS-500, CAB-O-SIL® TS-530, and CAB-O-SIL® TG-810G.AEROSIL® R8200 has a relatively high bulk density, making it useful forlowering the overall bulkiness of the formulation. Silica particlestreated with trimethoxy-octyl-silane are available from Degussa asAEROSIL® R805. Silica particles treated with hexadecyl silane,methyacryl-silane, and octamethyl-cyclo-tetra-siloxane are eachavailable from Degussa as AEROSIL′R816, AEROSIL® R711, and AEROSIL®R104, respectively. Silica particles treated with polydimethylsiloxaneare available from Cabot Corporation as CAB-O-SIL® TG-308F andCAB-O-SIL® TG-720. Silica treated with compounds having trimethylsiloxylmoieties is available from Tulco Inc. as TULLANOX 500. Additionally,silica particles treated with a combination of these and otherhydrophobic agents include, by way of example, AEROSIL® R 504, which hasa combination of triethoxy-propyl-amino-silane and hexamethyl-disilazaneas the surface treatment agents.

As referred to herein, silica particles include, but are not limited to,spherical particles. The silica particles can have grain-like or othernon-spherical shapes, and may be formed in small agglomerations.Preferably, the treated silica particles have an average surface area of130 m²/grams to 300 m²/grams and are coated with the hydrophobic agents.Preferably, the treated silica particles have an average particle sizein the range of 10 nm to 15 nm.

Representative concentrations of the hydrophobized silica particles inthe insulation composition range, for example, from about 35 phr toabout 70 phr. Generally, higher loads of hydrophobic silica particlescan be used than hydrophilic silica, since hydrophilic silica particleswill impart a greater increase to the viscosity of the insulationcomposition than an equal amount of hydrophobic silica particles.

The hydrophobized particles can be used alone or in combination withother materials affecting the ablative and mechanical properties of theinsulation. By way of example, suitable materials includepolybenzoxazole fibers, iron oxide, milled glass, carbon, ceramic clay,and the like.

The composition may also optionally include antioxidants to improve thelongevity of the cured elastomer. Examples of suitable antioxidants arediphenylamine reacted with acetone, available as BLE®-25 Liquid fromUniroyal Chemical; a mixture of mono-, di-, and tri-styrenated phenols,available as AGERITE® SPAR from B.F. Goodrich Chemical Co. Othersuitable antioxidants include polymerized1,2-dihydro-2,2,4-trimethylquinoline (AGERITE® RESIN D) and mixedoctylated diphenylamines (AGERITE® STALITE S), each of which isavailable from R.T. Vanderbilt Co.

Other optional ingredients include fillers that function as flameretardants. Flame retardants, or phosphate char forming additives, canbe used in lesser amounts than most other additives, which makes iteasier to formulate the insulation to possess, upon curing, goodmechanical properties. Both inorganic and organic flame retardants areexpected to be useful in the present invention. An example of an organicflame retardant is chlorinated hydrocarbon, available as DECHLORANE®, incombination with antimony oxide or hydrated alumina. Examples ofinorganic flame retardants are phosphate and phosphate derivatives,available as PHOSCHEK P/30® produced by Solutia, Inc.

An exemplary plasticizer for the inventive composition is TRILENE® 67A,which is a liquid EPDM elastomer available from Uniroyal.

Tackifiers may also optionally be used. Examples of suitable tackifiersare WINGTACK® 95 made by Goodyear Tire & Rubber Company and AKROCHEM®P-133 made by Akron Chemical Company.

Suitable cure activators include metal oxides, such as zinc oxide andmagnesium oxide (ELASTOMAG® 170, from Morton Chemical Co.), and stearicacid (including palmitic acid), which is available from Harwick StandardDistribution Corp. of Akron, Ohio.

It is also highly desirable to incorporate processing aids into theformulation in order to address the high stickiness of the compositions.An exemplary processing aid is STRUKTOL® HPS 11, which is a blend offatty acid derivatives, and STRUKTOL® WB 16, which is a mixture of fattyacid soaps. Both processing aids are available from Struktol Company. Asuitable concentration for the processing aids is about 2 phr.

Other ingredients well known in the art and/or suitable for use inrocket motor thermal insulation applications are intended to be includedwithin the scope of the present invention.

EXAMPLES

The following examples illustrate embodiments that have been made inaccordance with the present invention, as well as comparative examplesprepared for comparison purposes. The inventive embodiments are notexhaustive or exclusive, but merely representative of the many types ofembodiments which may be prepared according to this invention.

The compositions of Examples 1-8 and Comparative Examples A and B areset forth in Tables 1, 3, and 5. Tables 2 and 4 set forth the propertiesof the compositions subsequent to curing, which was conducted for 1 hourat 150° C. (320° F.). Unless otherwise indicated, all parts are in phr.

Example 1

A Brabender mixer having a net chamber volume of 350 cubic centimeterswas used for conducting a two-pass mix. The batch size was 300 grams.All of the ingredients except for the peroxide were added in the firstmix cycle, and mixing was performed at 30 rpm with a dust collectionsystem turned off. In the second mix cycle the dust collection systemwas turned on, and the peroxide curative was added and mixing wasperformed at 40 rpm.

Example 2

A laboratory scale Reliable Rubber & Plastics Machinery Company ModelR-260 internal mixture having a net chamber volume of 4260 cubiccentimeters was used for Example 2. A two-pass mix was used to make theformulation. The batch size was 3000 grams. All of the ingredientsexcept for the peroxide and SARET® 634 were added in the first mix cyclewith the dust collection system turned off, and mixed at 20 rpm mixingspeed. After the filler was incorporated, the dust collection system wasturned on and the mixer speed increased to 60 rpm to form masterbatch 1. Master batch 1 was dumped at a temperature between 110° C.(230° F.) and 121.1° C. (250° F.). The peroxide and SARET® 634 wereadded to master batch 1 in the second mix cycle and mixed at a mixingspeed of 40 rpm, then dumped at a temperature between 65.6° C. (150° F.)and 76.7° C. (170° F.). The dust collection system was on during theentire second mix cycle.

Example 3

To the fully compounded materials from Example 2 containing the peroxideand coagent, prior to crosslinking, was added extra TULLANOX® until 70parts per weight of filler was reached.

Comparative Example A

A Brabender mixer having a net chamber volume of 350 cubic centimeterswas used for conducting a two-pass mix. The batch size was 300 grams.All of the ingredients except for the peroxide were added in the firstmix cycle, and mixing was performed at 30 rpm with a dust collectionsystem turned off. In the second mix cycle the dust collection systemwas maintained off, and the peroxide curative was added and mixing wasperformed at 40 rpm.

TABLE 1 Comparative Example 1 Example 2 Example 3 Example A NORDEL ® 5555 55 55 1040 EPDM* NORDEL ® 15 30 30 15 2522 EPDM* TRILENE ® 30 15 1530 67A Liquid EPDM AGERITE ® 2 2 2 2 Resin D WINGTACK ® 7 7 7 7 95HI-SIL ® 233 — — — 45 TULLANOX ® 45 45 70 — 500 N-330 Carbon 1 1 1 1Black TZFD88-p 5 5 5 5 Zinc Oxide Stearic Acid — 1 1 — SARET ® 634 — 1010 — DI-CUP ® 10 19 10 10 10 40KE Total 170 181 206 170 *available fromDuPont Dow Elastomers of Beaumont, Texas., and contain 1,4-hexadiene(HD) as the diene monomer component.

TABLE 2 Comparative Example 1 Example 2 Example 3 Example A Mooneyviscosity (MU 43.8 55.8 85.4 69.5 at 100° C.; ASTM D1646) SpecificGravity 1.0866 1.1030 1.1489 1.0956 (ASTM D792) Ash Content (%) 31.4231.17 37.9 30.53 Shore A Hardness 61.2 77.4 81.8 71.0 (ASTM D2240) 100%Modulus (psi) 150 459 515 256 (ASTM D412) Tensile Strength (psi) 17802810 2610 1990 (ASTM D412) Elongation (%) 724 618 650 671 (ASTM D412)Tear Resistance (pli) 130 327 386 195 (ASTM D624)

From Table 1, it is understood that Examples 2 and 3 were prepared inaccordance with a preferred embodiment insofar as these examples containhydrophobic silica, peroxide curative and metallic curative coagent. InExample 1.

The composition was free of metallic curative coagent.

As shown in Table 2, Examples 2 and 3 respectively exhibited tensilestrengths of 2810 and 2610 psi, which are about 50% greater than thetensile strength of Example 1 and 30-40% greater than the tensilestrength of Comparative Example A. Further, Examples 2 and 3respectively exhibited tear resistances of 327 and 386 pli, which were 2to 3 times greater than the tear resistance of Example 1 and more than50% greater than the tear resistance of Comparative Example A.Furthermore, each of Examples 2 and 3 exhibited a respective stiffness,as measured by 100% modulus, of 459 psi and 515 psi. Generally, theinsulation of this invention that is highly tolerant to damage willexhibit a 100% modulus of at least 400 psi, more preferably at least 500psi, and a tear resistance of at least 300 pli, as measured by theabove-mentioned ASTM standards.

The reduced moisture sensitivity of insulation prepared in accordancewith this invention is demonstrated by FIG. 2. In FIG. 2, the moisturegain of Example 2 (designated by square data points) and ComparativeExample A (designated by diamond data points) were measured at 85%relative humidity over a period of more than 650 hours. The resultsshowed that Comparative Example A gained more than 3 times the amount ofmoisture over a 650-hour period than Example 2.

Also of interest is that the tensile strength and tear resistance ofComparative Example A were higher than found in Example 1. Thisreinforces the unexpected results obtained by this invention, since fromComparative Example A and Example 1 one of ordinary skill in the artwould have expected the replacement of hydrophilic silica (ComparativeExample A) with hydrophobic silica (Example 1) to adversely affectphysical properties. However, the inventors found that the synergisticeffect of improved physical properties and lower moisture sensitivitycan be realized by using the hydrophobic silica in combination with aperoxide curative and metallic curative coagent.

The ablative properties of the inventive formulation are illustratedfurther in connection with Examples 4-8 and Comparative Example B below.

Examples 4 and 8 and Comparative Example B

The two pass mix cycle described above in connection with Example 1 wasused to make Examples 4 and 8, except that mixing was performed in alaboratory scale Reliable Rubber & Plastics Machinery Company ModelR-260 internal mixture having a net chamber volume of 4260 cubiccentimeters. The batch size was 3000 grams.

Examples 5-7

A laboratory scale Reliable Rubber & Plastics Machinery Company ModelR-260 internal mixture having a net chamber volume of 4260 cubiccentimeters was used for Examples 5-7. Because of the bulkiness of thehydrophobic silica, a three-pass mix was used to make the formulations.The batch size for each phase of the mixing was 3000 grams. All of theingredients except for the peroxide and half of the hydrophobicparticles were added in the first mix cycle with the dust collectionsystem turned off, and mixed at 20 rpm mixing speed. After the fillerwas incorporated, the dust collection system was turned on and the mixerspeed increased to 60 rpm to form master batch 1. Master batch 1 wasdumped at a temperature between 110° C. (230° F.) and 121.1° C. (250°F.). Similarly, master batch 2 was mixed by adding the remainder of thehydrophobic filler to master batch 1, with the mixer speed set at 20 rpmand the dust collection system turned off. After the filler wasincorporated, the dust collection system was turned back on, and themixer speed was increased to 60 rpm. Master batch 2 was dumped at atemperature between 110° C. (230° F.) and 121.1° C. (250° F.). Theperoxide was added to master batch 2 in the third mix cycle and mixed ata mixing speed of 40 rpm, then dumped at a temperature between 65.6° C.(150° F.) and 76.7° C. (170° F.). The dust collection system was onduring the entire third mix cycle.

TABLE 3 Ex- Ex- ample 4 ample 5 Example 6 Example 7 Example 8 NORDELIP ® 55 55 55 55 55 NDR-4640** NORDEL IP ® 30 30 30 30 15 NDR-4520**NORDEL IP ® — — — — 30 NDR-3722p TRILENE ® 15 15 15 15 — 67A Liquid EPDMAGERITE ® 2 2 2 2 2 Resin D WINGTACK ® 7 7 7 7 7 95 CAB-O-SIL ® — 70 — —— TS-530 AEROSIL ® — — 70 — — R812S TULLANOX ® 45 — — 70 — 500 AEROSIL ®— — — — 70 R8200 Stearic Acid 1 1 1 1 1 STRUKTOL ® — — — — 2 HPS 11N-330 Carbon 1 1 1 1 1 Black TZFD88-p 5 5 5 5 5 Zinc Oxide SARET ® 63410 10 10 10 10 zinc dimethacrylate DI-CUP ® 6 10 10 6 4.5 40KE Total 177206 206 202 202.5 **available from DuPont Dow Elastomers of Beaumont,Texas., and contain ethylidene norbornene (ENB) as the diene monomercomponent.

TABLE 4 Ex- Ex- Ex- Ex- Ex- ample 4 ample 5 ample 6 ample 7 ample 8Mooney viscosity 56.0 106.4 111.4 92.3 85.9 (ML 1 + 4 at 100° C.) (ASTMD1646) Mooney scorch time 11.2 14.1 18.0 10.9 (MS + 1 at 115.6° C.)(ASTM D1646) Oscillating disk rheometer (160° C., 5° arc) (ASTM D2084)Ts2, min 1.2 1.0 1.1 1.0 2.7 ML, in.-lb. 12.3 14.5 14.0 12.5 16.9 MHR orMH (after 96.6 173.0 198.4 118.4 107.3 2 hrs) (in.-lb) Mc (90) (in-lb.)88.2 157.2 180.0 107.8 98.3 Tc (90) (min) 29.0 20.8 24.5 19.5 75.0Specific 1.0933 1.164 1.169 1.154 1.1531 Gravity(ASTM D792) Ash Content(%) 29.7 38.9 39.2 37.8 38.5 Shore A Hardness 68.2 87.8 86.8 84.4 85.1(ASTM D2240) 100% Modulus (psi) 369 918 943 492 519 (ASTM D412) TensileStrength (psi) 3100 2800 2810 2780 2710 (ASTM D412) Elongation (%) 649490 478 750 754 (ASTM D412) Tear Resistance (pli) 346 375 381 470 443(ASTM D624)

The Mooney viscosities of the formulations that contained 70 phr offiller were on the high side, but still within the experience base forconventional silica-filled EPDM insulation formulations. All of theinventive formulations exhibited excellent physical properties. Thelower stiffness of Example 4, 7, and 8 was attributed to its lowerperoxide levels.

TABLE 5 (Comparative Example B) DL1552A THERMAL INSULATION FORMULATIONParts Ingredient by Weight Buna EP T 3950 (Bayer Corp., Fiber, Additivesand 75 Rubber Division of Orange, Texas) NORDEL ®2722E (DuPont DowElastomers) 20 WINGTACK 95 (hydrocarbon resin) (Goodyear Tire and 7Rubber Co., Chemical Division of Beaumont, Texas) IRGANOX 1010(tetrakis[methylene-3-(3′,5′-di-tert-butyl- 1 4′-hydroxyphenyl)proprionate]methane) (Ciba Specialty Chemicals, Additives Division,Tarrytown, N.Y.) TRYCOL DA-6 (decyl polyoxyethylene alcohol) 0.5(Chemical Associates, Inc. of Copley, Ohio) Stearic acid (includingpalmitic acid) (Harwick Standard Distribution Corp. of Akron, Ohio)HiSil 233 (silica hydrate) (PPG Industries, Inc. of Lake 45 Charles,Louisiana) Aluminum oxide C (Al₂O₃) (Degussa Corporation of 0.3Ridgefield Park, N.J.) N330 carbon black (Columbian Chemicals Co. ofMarietta, 1 Ga.) KALENE 1300 (butyl gum elastomer) (Hardman Division 20of Harcros Chemicals, Inc. of Belleville, N.J.) HYPALON 20(chlorosulfonated polyethylene) (DuPont 5 Dow Elastomers) AGERITE ResinD (polymerized trimethyl 0.25 dihydroquinone) (R.T. Vanderbilt Co., Inc.of Buena Park, Ca.) TZFD-88p (zinc oxide dispersed in an EPDM binder) 2(Rhein Chemie Corp. of Trenton, N.J.) SP 1056 (bromomethyl alkylatedphenolic resin) 15 (Schenectady Int'l, Inc. of Schenectady, N.Y.) TotalParts by Weight 193.05

Examples 5-7 were subject to comparison testing against ComparativeExample B, which represents a conventional thermal insulationformulation known as DL1552A (see Table 5) and containing hydrophilicsilica, in a modified high-Mach char motor (see FIG. 3) fired with RSRMTP-H1148 (polybutadieneacrylic acid acrylonitrile (PBAN)-based)propellant. In the char motor test assembly, the propellant wascontained in a beaker 30. Low velocity test specimens were located atregion 32 upstream of the throat 34, medium velocity insulation testspecimens were located in the region 36, and high velocity insulationtest specimens were located in region 38. Generally, such a char testmotor assembly allows the location of a plurality of differentinsulation formulation test specimens about the circumference at any ofregions 32, 36, and 38, in a conventional manner.

The motor was fired for 11.49 seconds at an average pressure of 947 psi.The moisture insensitive Examples 5-7 exhibited better results,particularly in the high-velocity section of the char motor (averageMach Number from 0.04 to 0.07) compared to Comparative Example B. In thehigh-velocity region, Examples 5-7 respectively exhibited 25.4, 38.4,and 23.9 percent less ablation than Comparative Example B.

The ablative performance of the hydrophobic silica-filled insulation wasalso assessed by use of thermal flash testing. The specific thermalflash test was developed to evaluate materials used in the severeablative environment of the aft dome of a large solid rocket motor.Samples were exposed to high heat flux from calibrated quartz lampswhile air was forced over the samples to cause degradation. Thethickness of the samples was measured before and after thermal exposure.For the purposes of this experience, the materials were subjected to aheat flux of 40 cal/(cm²)(sec) and an exposure time of five seconds in awind tunnel. Subsonic air flow was available to remove pyrolysisproducts in order to maintain the desired incident heat flux at thespecimen surface. A bank of quartz lamps supplied the heat flux. Thetotal material losses for Examples 5-7 were 32.3 mils, 32.0 mils, and31.8 mils, respectively, compared to 55.5 mils for Comparative ExampleB. FIG. 4 shows a comparison of thermal flash test results for Example 2(triangles) and Example 3 (squares) compared to Comparative Example B(diamonds). The moisture insensitive insulation of this invention hadlosses that were 42 to 44 percent lower, and hence significantly better,than the standard conventional insulation.

The foregoing detailed description of the invention has been providedfor the purpose of explaining the principles of the invention and itspractical application, thereby enabling others skilled in the art tounderstand the invention for various embodiments and with variousmodifications as are suited to the particular use contemplated. Theforegoing detailed description is not intended to be exhaustive or tolimit the invention to the precise embodiments disclosed. Modificationsand equivalents will be apparent to practitioners skilled in this artand are encompassed within the spirit and scope of the appended claims.

What is claimed is:
 1. A rocket motor assembly comprising: a case havinga combustion chamber; a nozzle in operative relationship with the case;at least one solid propellant loaded in the combustion chamber of thecase; and an elastomeric rocket motor insulation interposed between thecase and the at least one solid propellant, the elastomeric rocket motorinsulation comprising: at least one ethylene-propylene-diene monomer(EPDM) rubber, the EPDM rubber being formed from an EPDM polymer curedwith at least one peroxide curative and at least one curative coagent,the at least one curative coagent comprising a polyfunctional metal saltfor forming organometallic cross-link bonds in the EPDM rubber; andfiller comprising at least one member selected from the group consistingof silica particles and silicate particles, the filler being treatedwith at least one hydrophobic surface agent.
 2. The rocket motorassembly of claim 1, wherein the at least one curative coagent comprisesat least one member selected from the group consisting of metal salts,zinc diacrylte and zinc dimethacrylate.
 3. The rocket motor assembly ofclaim 2, wherein the at least one curative coagent is present in aneffective amount to impart upon the elastomer rocket motor insulation,subsequent to cure, a 100% modulus of at least 400 psi and a tearresistance of at least 300 pli.
 4. The rocket motor assembly of claim 2,wherein the EPDM rubber constitutes 50 weight % to 70 weight % of theelastomeric rocket motor insulation.
 5. A rocket motor assemblycomprising: a case having a combustion chamber; a nozzle in operativerelationship with the case; at least one solid propellant loaded in thecombustion chamber of the case; and an elastomeric rocket motorinsulation interposed between the case and the at least one solidpropellant, the elastomeric rocket motor insulation comprising: at leastone elastomer, the at least one elastomer being formed from at least oneorganic polymer cured with at least one peroxide curative and at leastone curative coagent, the at least one curative coagent comprising apolyfunctional metal salt for forming organometallic cross-link bonds;and filler comprising at least one member selected from the groupconsisting of silica particles and silicate particles, the filler beingtreated with at least one hydrophobic surface agent.
 6. The rocket motorassembly of claim 5, wherein the at least one elastomer comprises atleast one member selected from the group consisting ofethylene-propylene-diene monomer (EPDM) rubber, natural rubber, butylrubber, butadiene-styrene copolymer rubbers, nitrile rubbers, neoprenerubbers, polybutadiene rubbers, and polyisoprene rubbers.
 7. The rocketmotor assembly of claim 5, wherein the at least one elastomer comprisesethylene-propylene-diene monomer (EPDM) rubber.
 8. The rocket motorassembly of claim 5, wherein the at least one curative coagent comprisesat least one member selected from the group consisting of metal salts,zinc diacrylate and zinc dimethacrylate.
 9. The rocket motor assembly ofclaim 8, wherein the at least one curative coagent is present in aneffective amount to impart upon the elastomeric rocket motor insulation,subsequent to cure, a 100% modulus of at least 400 psi and a tearresistance of at least 300 pli.
 10. The rocket motor assembly of claim8, wherein the at least one elastomer constitutes 35 weight % to 80weight % of the elastomeric rocket motor insulation.
 11. A method formaking a rocket motor assembly, comprising: cross-linking at least oneethylene-propylene-diene monomer (EPDM) rubber with at least oneperoxide curative and at least one curative coagent in the presence of afiller to form an elastomeric rocket motor insulation, the at least onecurative coagent comprising a polyfunctional metal salt for formingorganometallic cross-link bonds in the at least one EPDM rubber, thefiller comprising at least one member selected from the group consistingof silica particles and silicate particles, the filler being treatedwith at least one hydrophobic surface agent; and preparing a rocketmotor assembly containing the elastomeric rocket motor insulation, therocket motor assembly comprising a case having a combustion chamber, anozzle in operative relationship with the case, at least one solidpropellant loaded in the combustion chamber of the case, and theelastomeric rocket motor insulation interposed between the case and theat least one solid propellant.
 12. The method of claim 11, wherein theat least one curative coagent comprises at least one member selectedfrom the group consisting of metal salts, zinc diacrylate and zincdimethacrylate.
 13. The method of claim 11, wherein the at least onecurative coagent is present in an effective amount to impart upon theelastomer rocket motor insulation, subsequent to cure, a 100% modulus ofat least 400 psi and a tear resistance of at least 300 pli.
 14. Themethod of claim 11, wherein the at least one EPDM rubber constitutes 50weight % to 70 weight % of the elastomeric rocket motor insulation. 15.A method for making a rocket motor assembly, comprising: cross-linkingat least one organic polymer with at least one peroxide curative and atleast one curative coagent in the presence of a filler to form anelastomeric rocket motor insulation, the at least one curative coagentcomprising a polyfunctional metal salt for forming organometalliccross-link bonds, the filler comprising at least one member selectedfrom the group consisting of silica particles and silicate particles,the filler being treated with at least one hydrophobic surface agent;and preparing a rocket motor assembly containing the elastomeric rocketmotor insulation, the rocket motor assembly comprising a case having acombustion chamber, a nozzle in operative relationship with the case, atleast one solid propellant loaded in the combustion chamber of the case,and the elastomeric rocket motor insulation interposed between the caseand the at least one solid propellant.
 16. The method of claim 15,wherein the at least one organic polymer comprises at least one memberselected from the group consisting of ethylene-propylene-diene monomer(EPDM) rubber, natural rubber, butyl rubber, butadiene-styrene copolymerrubbers, nitrile rubbers, neoprene rubbers, polybutadiene rubbers, andpolyisoprene rubbers.
 17. The method of claim 15, wherein the at leastone organic polymer comprises ethylene-propylene-diene monomer (EPDM)rubber.
 18. The method of claim 15, wherein the at least one curativecoagent comprises at least one member selected from consisting of metalsalts, zinc diacrylate and zinc dimethaclylate.
 19. The method of claim18, wherein the at least one curative coagent is present in an effectiveamount to impart upon the elastomeric rocket motor insulation,subsequent to curing, a 100% modulus of at least 400 psi and a tearresistance of at least 300 pli.
 20. The method of claim 18, wherein theat least one organic polymer constitutes 35 weight % to 80 weight % ofthe elastomeric rocket motor insulation.